Enhanced oil recovery refers to the process of producing liquid hydrocarbons by methods other than the conventional use of reservoir energy or water floods which utilize reservoir repressurizing schemes with injected water. On average, conventional production methods will produce from a reservoir about 30% of the initial oil in place. The remaining oil, nearly 70% of the initial resource, is a large and attractive target for enhanced oil recovery methods.
Waterflooding recovers oil by the water moving through the reservoir as a bank of fluid that displaces the oil ahead of it. The recovery efficiency of a waterflood is largely a function of the sweep efficiency of the flood and the ratio of the oil and water viscosities. Sweep efficiency is a measure of how well the water has come in contact with the available pore space in the oil-bearing zone. Gross heterogeneities in the rock matrix lead to low sweep efficiencies. Fractures, high-permeability streaks, and faults are examples of gross heterogeneities. Homogeneous rock formations provide the optimum setting for high sweep efficiencies.
The overall recovery efficiency of any fluid displacement process depends upon the macroscopic displacement efficiency and the microscopic displacement efficiency. The macroscopic displacement efficiency is a measure of how well the displacing fluid has come in contact with the oil-bearing parts of the reservoir. The microscopic displacement efficiency is a measure of how well the displacing fluid mobilizes the residual oil once the fluid has come in contact with the oil.
The microscopic displacement efficiency is affected by several factors, including interfacial and surface tension forces, wettability, capillary pressure, and relative permeability. The interfacial tension between two fluids represents the amount of work required to create a new unit of surface area at the interface. The interfacial tension can also be thought of as a measure of the immiscibility of two fluids. When certain chemical agents are added to an oil-brine system, it is possible to reduce the interfacial tension by several orders of magnitude, thereby greatly improving their miscibility.
The tendency for a solid to prefer one fluid over another is called wettability. Wettability is a function of the chemical composition of both the fluids and the rock. Rock surfaces can be either oil-wet or water-wet, depending on the chemical composition of the fluids. The degree to which a rock is either oil-wet or water-wet is strongly affected by the adsorption or desorption of constituents in the oil phase. Large, polar compounds in the oil phase can absorb onto solid surfaces leaving an oil film that may alter the wettability of the surface.
Chemical flooding relies on the addition of one or more chemical compounds to an injected fluid such as water, to either reduce the interfacial tension between the reservoir oil and the injected fluid, or to improve the displacement efficiency of the injected fluid. There are three general methods in chemical flooding technology. The first is polymer flooding, in which a large molecular weight component is used to increase the displacing fluid viscosity. This leads to improved displacement efficiencies in the reservoir. The second and third methods, micellar-polymer and alkaline flooding, make use of chemicals that reduce the interfacial tension between an oil and a displacing fluid.
The addition of large-molecular-weight molecules called polymers to an injected water may increase the effectiveness of a conventional waterflood. Polymers are sometimes added to the water in concentrations ranging from 250 to 2000 parts per million (ppm). A polymer solution is more viscous than a brine without polymer. In a flooding application, the increased viscosity may alter the mobility ratio between the injected fluid and the reservoir oil. The improved mobility ratio may lead to better displacement efficiencies and thus higher oil recoveries.
The micellar-polymer process uses a surfactant to lower the interfacial tension between the injected fluid and the reservoir oil. A surfactant migrates to the interface between the oil and water phases and helps make the two phases more miscible. Interfacial tensions can be reduced from ˜30 dyne/cm, found in typical waterflooding applications, to 10-4 dyne/cm with the addition of as little as 0.1-5.0 wt % surfactant to water-oil systems. As the interfacial tension between an oil phase and a water phase is reduced, the capacity of the aqueous phase to displace the trapped oil phase from the pores of the rock matrix increases. The reduction of interfacial tension results in a shifting of the relative permeability curves such that the oil will flow more readily at lower oil saturations.
When an alkaline solution is mixed with certain crude oils, surfactant molecules are formed. When the formation of surfactant molecules occurs in situ, the interfacial tension between the brine and oil phases can be reduced. The reduction of interfacial tension causes the microscopic displacement efficiency to increase, which may increase oil recovery. Alkaline substances that have been effectively used include sodium hydroxide, sodium orthosilicate, sodium metasilicate, sodium carbonate, ammonia, and ammonium hydroxide.
However, enhanced oil recovery methods face numerous challenges. This is largely due to the complexity of an oil producing well. Factors that affect the impact of a waterflooding system start with the geography and type of rock formation and include the chemical composition of the formation (e.g. sandstone versus limestone), the heterogeneity of the formation, the porosity of the rock, and the formation's macroscopic features (e.g. presence of cracks, fissures and the like). The characteristics of the injection water itself are equally important. These include temperature, viscosity, pH, salinity and osmotic strength. The combined characteristics of the rock formation, the injection water, and the oil all define how the oil in the formation will interact and/or react with the injection water and thus define key metrics such as surface wettability, displacement efficiencies, viscous stripping, oil film flow, oil saturation and buoyancy forces. So, it is clear that enhancing oil recovery in existing production wells is an extremely complex process and no single injection water composition or waterflooding method is likely to be efficient and cost effective for every, or even most, rock formations treated.
Thus, there remains a significant need in the field of enhanced oil recovery for producing efficient and cost effective aqueous displacement injection compositions, and methods for applying said compositions to subterranean rock formations, wherein the compositions and methods are specifically designed for the specific rock formation being treated. There also remains a need for adaptive waterflooding methods, that to continue to maximize oil recovery during the dynamic changes that occur in any rock formation, waterflooding procedure.